Unless you've got some urge to take half the day and semi-scenic backroads and towns, it is by far much better to take I-10 W to I-37 S.
And I would take tx 358 as you approach CC, over to N. Padre Island and then up Mustang Island 361 to Port A.
(I highly recommend avoiding the ferries that you'd have to wait-take if cutting over from CC through Portland/Aransas Pass - 181/35)
As someone that makes this trip regularly I would disagree, with all due respect. Once you leave Seguin on Hwy 123 you will have one light in Stockdale and two lights in Kenedy until you get to Sinton.
Just outside of Sinton there will be a TXDOT electronic sign giving you the wait time for the ferry. If it is under 30 minutes continue through Taft (Speed trap) and Aransas Pass to the ferry. You can also check ferry wait times on line from TXDOT.
If the wait is longer, before reaching Sinton take Hwy 77 to I 37 then follow 358 to Mustang Island.
In my experience it is actually shorter and a few minutes quicker than through San Antonio. It is without a doubt more scenic and enjoyable. (my opinion)
The Interstate is an "easier" drive but definitely not any quicker, although you will miss any real traffic going through San Antonio. If you choose this route take 130 to I10 to I410 to I37 South. I would definitely use this route if traveling at night.
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